[FFmpeg-devel] [PATCH 1/2] AAC: fix strict aliasing violation in parser

[Måns Rullgård]

"Eli Friedman" <eli.friedman at gmail.com> writes:

> On Sun, Dec 14, 2008 at 12:53 PM, Mans Rullgard <mans at mansr.com> wrote:
>> -    uint8_t tmp[8];
>> +    union {
>> +        uint64_t u64;
>> +        uint8_t  u8[8];
>> +    } tmp;
>>
>> -    AV_WB64(tmp, state);
>> -    init_get_bits(&bits, tmp+8-AAC_HEADER_SIZE, AAC_HEADER_SIZE * 8);
>> +    tmp.u64 = be2me_64(state);
>> +    init_get_bits(&bits, tmp.u8+8-AAC_HEADER_SIZE, AAC_HEADER_SIZE * 8);
>
> As far as I can tell, this patch does nothing except trick gcc...
> uint8_t isn't technically guaranteed to be a character type, but in
> practice, it always is, so uint8_t loads should always be safe.

That is not the issue.  While it is safe to access a non-char type
through a pointer to char, the converse is not true.  However,
Corrigendum 3 (2007) to the C99 standard adds this text:

   If the member used to access the contents of a union object is not
   the same as the member last used to store a value in the object,
   the appropriate part of the object representation of the value is
   reinterpreted as an object representation in the new type as
   described in 6.2.6 (a process sometimes called "type
   punning"). This might be a trap representation.

I am uncertain whether this also covers accesses through a pointer to
a union member cast to a different type of pointer.

-- 
M?ns Rullg?rd
mans at mansr.com